Ajax: don’t test jQuery.ajax() on beforeunload in Chrome 31

Chrome 31 doesn't fire Ajax requests in beforeunload event handler.
There is no way for us to workaround it and it's been fixed in Chrome 32
so let's just blacklist Chrome 31 as long as it's in TestSwarm.
See https://code.google.com/p/chromium/issues/detail?id=321241

(cherry-picked from fd0bce07584b73fb755574c4d462e0b175d1c6b6)